Acquire lock before iterating over list of field trials
Acquire lock before iterating over list of field trials

The itteration is performed in the browser process
when the list of field-trial settings is passed to
the renderer.  I failed to acquire the lock prior
to the itteration, and it appears that some code
is deciding on a field trial VERY late in browser
startup and racing against this render-spawn
operation.  The code in FieldTrial is wrong
(but fixed here), but I'll still need to
track down the racing thread that is performing
such a late FieldTrial definition (the results
of which will not be passed to some renderers).

BUG=49409
R=huanr
Review URL: http://codereview.chromium.org/3134015

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@56360 0039d316-1c4b-4281-b951-d872f2087c98

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